In recent years, devices having a micro structure and produced by a micro-machining technology, which is sometimes called “MEMS (Micro Electro Mechanical Systems) technology”, have been put into applications in a variety of fields.
The MEMS devices include such types as a MEMS switch, a MEMS capacitor, a MEMS sensor, and so on for a high-frequency circuit. For example, the MEMS switch has an advantageous feature, as compared with a conventional semiconductor switch, such as a small loss, high insulating properties, and good distortion properties.
As a conventional technology, Japanese Laid-open Patent Publication No. 2005-293918 proposes a MEMS switch in which a movable portion is formed on a substrate, and a contact provided to the movable portion makes contact with a contact electrode provided in a fixed manner relative to the substrate.
In a MEMS device, the movable portion is fabricated by using, for example, an ordinary SOI wafer and applying a D-RIE process only to the active layer (device layer) thereof. Alternatively, the movable portion is sometimes fabricated by laminating Poly-Si, Poly-SiGe, or the like on the wafer as a device layer, and applying an etching process or removing a sacrifice layer. Depending on the MEMS device, there is also a method to fabricate the movable portion by bonding a layer to a base wafer, and applying a D-RIE process. Among these processes, the process of removing the sacrifice layer to make a structure laminated on lower and upper layers of the sacrifice layer movable is called a surface MEMS process.
FIG. 13 is a plan view illustrating an example of a MEMS switch 80j, and FIG. 14 is a cross sectional view of the MEMS switch 80j illustrated in FIG. 13 taken along a line J-J.
Referring to FIGS. 13 and 14, the MEMS switch 80j includes a substrate 81, a lower contact electrode 82, an upper contact electrode 83, a lower driving electrode 84, an upper driving electrode 85, and so on, all of which are formed on the substrate 81. The lower contact electrode 82 and the lower driving electrode 84 are integrally provided to a movable portion KBj that constitutes a cantilever.
An SOI substrate is used as the substrate 81. The movable portion KBj is formed by cutting off the active layer of the SOI substrate by a slit SL. The lower contact electrode 82 and the lower driving electrode 84 are formed on the active layer by plating.
When a driving voltage is applied between the upper driving electrode 85 and the lower driving electrode 84, an electrostatic attractive force is generated therebetween, with which the lower driving electrode 84 is attracted toward and moved to the upper driving electrode 85. In this way, the movable portion KBj and the lower contact electrode 82 that are integrated with the lower driving electrode 84 move, and the lower contact electrode 82 touches the upper contact electrode 83 so that the contacts close. At this time, if the driving voltage is set at zero, the contacts return to the positions separated from each other due to the elasticity of the movable portion KBj.
The MEMS switch 80j described above has a structure in which a cavity is present below the lower surface of the movable portion KBj, and only one end of the movable portion KBj is connected to and supported by the substrate 81. The movable portion KBj is capable of bending upward and downward with the supported portion serving as a fulcrum point.
During a process of manufacturing the MEMS switch 80j, when an electrode having a coefficient of thermal expansion larger than that of the base material is laminated on the upper surface of the movable portion KBj, and when the temperature goes down to a room temperature, a stress is generated to cause the movable portion KBj to warp upwardly. When a sacrifice layer such as SiO2 is further laminated thereon, the laminated sacrifice layer generates a stress which causes the movable portion KBj to warp downwardly. Although the warpage of the movable portion KBj caused by the electrode is small, for example, about 0.3 μm, the downward warpage of the movable portion KBj caused by the sacrifice layer sometimes becomes, for example, about 1 μm of which the influence is great.
In other words, during a process of manufacturing the MEMS switch 80j, a half etching of the sacrifice layer is performed to form the contact of the upper contact electrode 83. However, if the movable portion KBj largely warps, the adjustment or the control of the etching depth can not be accurately performed. For this reason, the accuracy of the interelectrode gap between the contact of the upper contact electrode 83 and the lower contact electrode 82 after the sacrifice layer is removed is worsened. Accordingly, desired switching properties may not be obtained.
In addition, if large downward warpage of the movable portion KBj is caused, there are sometimes cases where the upper surface portion of the slit SL may not be completely filled with the sacrifice layer. In such a case, the resist or polymer may infiltrate into a gap of the slit SL during a post-process, which makes it difficult to remove such a substance by cleaning, and reduces yields.